In today's systems, before a wireless device, which may also be called a user equipment (UE), begins communicating with the network, it must detect and decode synchronization signals sent out by access nodes (ANs) within the network. After decoding the network information sent through a received synchronization signal, the device is able to communicate with the corresponding AN to start a random-access procedure. Such synchronization signals are generally sent omnidirectionally by current systems such as, for example, LTE systems operating at below-6 GHz bands. Thus, wherever a UE requires synchronization, such signals may be detectable without requiring multi-antenna (spatial) processing.
However, transmission of synchronization signals enabling initial access of UEs becomes a challenge in scenarios requiring the deployment of antenna arrays with a massive number of antenna elements, possibly both at ANs and UEs, especially in mm-wave bands. Such scenarios are envisioned to be relevant in the next generation. Specifically, for example, such scenarios may be relevant in 5G wireless communication systems. Due to the inherent beam narrowness and severe mm-wave propagation conditions in such systems, energy radiation is concentrated in a very small area during a transmission interval. Thus, the detection of synchronization signals by UEs in such scenarios requires achieving spatial alignment between transmit and receive beam directions.
To achieve spatial alignment between transmit and receive beams, a beam sweep procedure is adopted, where ANs transmit synchronization signals in different beam directions, one at a time, while UEs search for synchronization signals over different beam directions.